We have done three consecutive August trips (2012, 2013 and 2014....the last one was a two-week stint) and in addition to what's already been noted here, moisture-wicking t's (like Dri-fit) and light bottoms (no jean shorts

) are a must. Long hair worn up will help immensely. DH also likes his misting fan.
Sunscreen, sunscreen, sunscreen. You can sweat that stuff off in a heartbeat. And if you're lily white like we are, buy the highest SPF you can find. We never use less than 100 and can still manage to burn.
We're from the north, so we feel the heat and humidity REALLY badly (for the record, yesterday it was -32 degrees celcius here, which is -25.6 degrees Farenheit for my American friends). We don't really get used to Florida's oppressive summer temperatures, no matter how long we stay...that's why we incorporate as much "wet" time into our August trips as we can (pool, waterparks, etc.).